Optimization and mechanisms of rapid adsorptive removal of chromium (VI) from wastewater using industrial waste derived nanoparticles

Nowadays, the existence of metal ions in the environment like chromium (VI) is of significant worry because of its high toxicity to many life forms. Therefore, in this study, an inexpensive and eco-friendly nano-adsorbent was produced from the waste of drinking water industry for effective elimination of Cr (VI) from wastewater. The mineralogical and morphological characterization and compositions of the bulk and nano- adsorbents were performed. The adsorption capabilities of nWTRs for Cr(VI) under different experimental conditions of adsorbent dosage, time, Cr (VI) concentration, solution pH, and competitive ions were investigated. The nWTRs adsorbent exhibits very rapid adsorption potential (92%) for Cr (VI) within the first 15 min. Langmuir model showed high predictive capability for describing Cr (VI) sorption equilibrium data. The estimated maximum sorption capacity (qmax) of nWTRs and bWTRs was found to be 40.65 mg g−1 and 2.78 mg g−1 respectively. The sorption kinetics data of Cr (VI) were perfectly fitted to the model of second-order kinetics. High immobilization capability of nWTRs for sorbed Cr (VI) is evident as most of adsorbed Cr (VI) was associated with the residual fraction. The nWTRs efficiency of Cr (VI) removal from wastewater using batch and column techniques were 98.12 and 96.86% respectively. Electrostatic interactions, outer sphere complexation and pore filling are the main mechanisms suggested for binding of Cr(VI) with functional groups of nWTRs. This study demonstrates that the green low-cost nWTRs have the potential to decontaminate industrial wastewater effluents containing Cr (VI).


Sorption kinetics.
Sorption kinetic studies were conducted in 50 ml centrifuge tubes containing 100 mg/10 ml of nWTRs and Cr(VI) with 350 mgl −1 initial concentration. The mixtures were shaken (end-overend shaker) for various time intervals (5,30,60,240,480, and 1440 min) at three different pH levels (5, 7 and 9). Solution pH was adjusted with hydrochloric acid (HCl) and sodium hydroxide (NaOH) with concentrations of 0.1 mol l −1 . The liquors were centrifuged and filtered (0.45 um Millipore filter). Using inductively coupled plasma spectrometry (ICPS), the supernatant was analyzed for Cr(VI), and the obtained data were fitted to five kinetic models. Sorption isotherms. The Cr(VI) sorption equilibrium studies were determined using two different particle sizes of WTRs (2 mm and less than 100 nm) at different concentrations (0, 5 20, 40.80 and 160 mgl −1 ) in a background electrolyte of 0.005 M KNO 3 . Solutions obtained were placed in contact with WTRs materials in 50 ml capped polyethylene tubes. The bulk WTRs-Cr(VI) mixtures were equilibrated on a slowly rotating rack for 1440 min at normal pH (7.2), and then removed for centrifugation (4000 rpm) for 15 min, and the data obtained were fitted into different isotherms models. The solid nWTRs-Cr(VI) mixtures after sorption experiments were recovered and examined for sorbed Cr(VI) utilizing SEM, TEM and FTIR analysis.
To study the impact of nWTRs dosage on the Cr(VI) removal, 20, 50, or 100 mg of nWTRs sample were added to 10 ml portions of Cr (VI) with concentration of 200 mgl −1 . Competitive adsorption tests in batch (single and multi-ions systems) were performed using nWTRs and solutions containing equal amounts of As (V), Hg (II), and Cr (VI) (multi-ion system) with different concentrations (0, 5, 20, 40.80 and 160 mgl −1 ) in background electrolyte (0.005 M KNO 3 ). The suspensions were equilibrated for 1440 min.

Results and discussion
Characteristics and chemical composition of nWTRs. SEM and EDX are powerful tools for examining the structure of surface morphology and the elemental features of the sorbents 28 . The particles morphology and elemental contents of nWTRs and Cr(VI) loaded nWTRs are shown in Fig. 1. The SEM image of the nWTRs sample (Fig. 1A) revealed that the nanoparticles were spherical, with particle sizes < 100 nm in diameter. EDX elemental analysis (Fig. 1C) exhibited the major elements of nWTRs (iron, silicon, calcium, and aluminum) with small amounts of potassium, sulfur, titanium, and manganese. The SEM image of Cr(VI) loaded nWTRs (Fig. 1B) revealed a coating layer of Cr(VI) adsorbed on the nWTRs surface, indicating that the reaction occurred on the nWTRs surface. According to the EDX analysis (Fig. 1D), a Cr (VI) peak (11.80%) is noticed among the elements detected in Cr(VI)-loaded nWTRs.
The TEM image of the nWTRs and Cr(VI) loaded nWTRs (Fig. 1E,F) revealed that the nWTRs are somewhat agglomerated with particle sizes ranging from 18.1 to 76.1 nm. Surface functional groups of nanoscale WTRs. The sorbents characterization by Fourier transmission infrared (FTIR) spectra reveals the functional group in the sorbent's structure 29 which is crucial for understanding mechanisms of metals removal by the solid surfaces. FTIR spectroscopy results of nWTRs and Cr(VI) loaded nWTRs are presented in Fig. 2A. The bands detected at 3619 cm −1 , 3417 cm −1 and 1636 cm −1 in the FTIR spectrum of nWTRs ( Fig. 2A lower) are assigned to dangling O-H bonds, the HO-H stretching and bending vibrations respectively 22,30 . Additionally the observed bands at 1449 cm −1 ,1091 cm −1 , and 601 cm −1 are assigned to Fe OH bending vibration-modes of feroxyhyte 30,31 and bending vibration of Al single bond OH group 32,33 . After Cr(VI) adsorption, the bands and peaks between 3417 and 4012 cm −1 (referred to the dangling O-H bonds and O-H bending vibration) as well as the peak at 1793 cm −1 completely disappeared. In addition, shifting of the peak at 1626 cm −1 to higher wavenumber (1646 cm −1 ) and shifting of the peak at 1091 cm −1 referred to O-Al-O bending vibration was shifted to lower wave number(1016 cm −1 ) were observed. Disappearance and shifting of peaks and bands suggest molecular interaction and adsorption of Cr(VI) on nWTRs through functional groups 34   ity sorbed onto nWTRs compared to that sorbed onto μWTRs and bWTRs, the distribution of Cr(VI) in the fractions of the sorbents studied was performed using the widely employed sequential extractions(fractionation) scheme 35,36 . According to this scheme, the unstable Cr (VI) in the non-residual fraction is more mobile than the Cr(VI) in the residual (RS) fraction. Thus, correlating Cr(VI) data with sorbent fractions would determine the mobility of Cr(VI) bound to various sorbent fractions. Figure 2B. Clearly exhibits that Cr(VI) adsorbed by bWTRs was mostly correlated with the more mobile Non-RS fraction (61.67%) while 83.64% of Cr(VI) on nWTRs was related to the less mobile fraction (RS). These variations are referred to the larger surface area and higher adsorption capacity of nWTRs.
Adsorption equilibrium study. Adsorption isotherm models are extremely useful in determining adsorption capacity of the sorbents and in selecting the most suitable sorbent under optimal experimental conditions 37,38 . The experimental adsorption equilibrium data of the present study were analyzed using six adsorption isotherm models ( Table 1). The determination coefficient (R 2 ) and the standard error of estimate (SE) values presented in (Table 1) are indicators of how well the model fits the data. The highest R 2 value and the lowest SE value of Langmuir model (Table 1) signalize the best fit of the model to Cr(VI) adsorption data. The Lowest SE values of 0.00062 and 0.000014 (Table 1 and Fig. 4A) of Cr(VI) adsorption onto bWTRs and nWTRs sorbents respectively affirmed that Cr(VI) adsorption onto the studied sorbents fitted best to Langmuir model. The calculated maximum sorption capacity (q max ) of nWTRs and bWTRs were found to be 40.65 mg and 2.78 mg Cr (VI) g −1 respectively. The calculated q max value of nWTRs was 15 times higher than that of bWTRs.

Effect of the varying different parameters on Cr (VI) adsorption. Initial Cr (VI) concentration.
The influence of initial Cr (VI) concentration at a range of 5 to160 mgl −1 on adsorption capacity of nWTRs is shown in Fig. 3A. The results show that increasing Cr (VI) initial concentration has led to increasing Cr (VI) adsorption onto nWTRs adsorbent. The Cr (VI) adsorbed by nWTRs increased from 999 to 31,974 μgg −1 with increasing Cr (VI) initial concentration from 5 to 160 mgl −1 due to the increased mass transfer driving force which encouraged the adsorbate transport from the solution to the adsorbent surface 39 and enhanced the interaction between adsorbent and adsorbate 40 .
Competitive ions. The competition effects of the amounts of Cr (VI) adsorbed by nWTRs in presence or absence of competing cations such as As and Hg at concentrations equal to Cr (VI) concentration were investigated in single and multi-element systems (Fig. 3A). As shown, the power of nWTRs for removing Cr (VI) was www.nature.com/scientificreports/ highly affected as a result of As and Hg presence in the solution. For instance, at high Cr (VI) initial concentration, Cr (VI) removal dramatically decreased from 99 to 24% due to competition between Cr and As/Hg cations for available sorption sites of nWTRs (Fig. 3A). Similarly, Jain et al. 41 indicated that the adsorption capacity of sunflower plant biomass-based carbons for Cr (VI) was reduced in binary and tertiary multi-element systems because of the progressive interference and competition of cations for binding sites on the adsorbent. Adsorbent dosage. The effect of nWTRs dose (20-100 mg) on Cr(VI) adsorption was studied. Increasing nWTRs dosage from 20 to 100 mg increased the adsorption efficiency of the adsorbent and the adsorbed amount of Cr(VI) increased from 8000 to 17,200 μgg −1 (Fig. 3B). This is because of the greater surface area and the availability of larger number of adsorption sites for HCrO 4 − and Cr 2 O 4 − ions 42 . Therefore, the dose of 100 mg (nWTRs) was used in the optimization studies.
Solution pH. The Cr (VI) in solution is mostly found in the forms of H 2 CrO 4 , HCrO 4 − , Cr 2 O 7 2− , CrO 4 2− depending on Cr(VI initial concentration and pH as shown in Fig. 3E. The pH value of sorbent is a significant parameter for metal adsorption [43][44][45] . Thus, the impact of pH solution on Cr(VI) removal by nWTRs in the pH range (3-11) was examined. Figure (3C) clearly shows that Cr(VI) adsorption on to nWTRs is reduced significantly with increasing pH until it reaches pH 11. For better understanding of Cr(VI) adsorption mechanisms, the point zero charge (pHzpc) of nWTRs was determined and found to be 7.43 (Fig. 3F). At pH values lower than 7.43 the nWTRs surface is positively charged whereas at higher pH values than7.43, the nWTRs surface is negatively charged 46,47  ), and nWTRs surface is positively charged, strong electrical interactions between Cr(VI) species and positively charged sites on nWTRs surface are taking place and as a result the adsorption capacity of nWTRs increased (Fig. 3C). Conversely, as the pH of the nWTRs increased above its zero point charge (pH > pHzpc), the Table 1. The parameters of isotherms models for Cr(VI) adsorption onto the two particle sizes of WTRs.

Adsorption isotherms bWTRs nWTRs
Freundlich q e = K F C  Modelling adsorption kinetics. Five models were utilized to model the Cr(VI) adsorption kinetics data by nanoscale WTRs at three pH values (5, 7, and 9) 53 . The adsorption kinetics models used were evaluated to determine the best fit model that could reproduce the results. Table 2 shows the kinetic models tested and its parameters. Generally, higher R 2 values and lower SE values better describe the kinetic model of adsorption process. It can be clearly seen ( Table 2) that kinetics data of Cr(VI) adsorption by nanoscale WTRs at pH values (5, 7, and 9) are best described by the second-order model based on the lowest standard errors (SE) values and the highest determination coefficient (R 2 ) of the second-order model ( Table 2 and Fig. 4B). (iv) Reduction process At a lower pH, Cr(VI) may be reduced to Cr(III) 57 , which could then be chelated with Al/Fe oxides on the surface of nWTRs. www.nature.com/scientificreports/ The feasibility of nWTRs application for Cr(VI) removal from wastewater. The ability of nWTRs to efficiently remove Cr(VI) from actual wastewater was evaluated using batch and packed-bed column techniques. Samples from wastewater discharged from the food processing plant were collected in polyethylene bottles. The pH, electric conductivity (EC), Cr(VI) concentration, major and minor elements, of the wastewater samples were all measured and are presented in Table S1. All wastewater samples (one Liter) were spiked with 100 mgl −1 Cr(VI) solution. A 100 mg nWTRs was added to each wastewater sample. The mixture was shaken for 120 min, then centrifuged and the supernatant solutions were filtered and analyzed.
Packed-bed column technique. The columns were made of plastic (UV stabilized PVC) with 25 cm length and internal diameter of 1.2 cm. Each column was filled with 600 mg nano-WTRs and sand at a specific ratio (5 g fine sand and 5 g coarse sand). The structure of columns bottom consist of a wool layer and a filter  www.nature.com/scientificreports/ paper to preserve the solid contents of the column. Wastewater effluents spiked with Cr(VI) (100 mgl −1 ) were infiltrated across the column downward at low flow rate of 2 ml/min. The leachate was obtained at regular time intervals and analyzed. The efficiency of nWTRs using a real chromium contaminated wastewater sample, in batch (shaking) and packed-bed column were 98.12 and 96.86 respectively (Fig. 6).

Comparison of adsorption capacity of nWTRs and other adsorbents for Cr(VI) removal.
The maximum adsorption capacity (qmax) of nWTRs for chromium is compared with different adsorbents (Table 3). It can be seen that the nanostructured WTRs outperformed all the listed adsorbents in Cr(VI) removal from aqueous solutions.

Conclusions
A low-cost nano-adsorbent (nWTRs) produced from byproducts of water industry has proven to be highly effective in removing Cr(VI) from contaminated wastewater. The linearized Langmuir model was statistically superior to the other models tested and showed excellent ability to model Cr(VI) adsorption data with good accuracy. The calculated maximum sorption capacity (qmax) of nWTRs (40.65 mgg −1 ) for Cr(VI) was 15 times higher than qmax of bulk WTRs (2.78 mgg −1 ). The second-order model best described Cr(VI) sorption kinetics data. FTIR and SEM-EDX spectra analysis suggests that the hydroxyls may act as the surface active sites for Cr(VI) binding to nWTRs surfaces. The efficiency of nWTRs in removing Cr (VI) from wastewater effluents using batch  www.nature.com/scientificreports/ and column techniques were 98.12% and 96.86% respectively. It is, therefore, suggested the use of nWTRs as a powerful and rapid sorbent for elimination of Cr (VI) from industrial wastewater effluents.

Data availability
All data generated or analyzed during this study are included in this published article and its supplementary information files.